Vehicle transmissions have been provided in recent years with an increasing number of individually selectable speed ratios. Seven or more speed ratios may be provided, and by doing so many factors relating to vehicle performance and economy can be enhanced.
Control strategies for automatic transmissions are highly complex, taking into account numerous external and internal factors to determine speed ratio selection and the timing of a speed ratio change. These strategies are generally incorporated within a shift map which comprises a decision matrix for all circumstances of use of the transmission. Implementing a new shift map to accommodate a new circumstance can be an extended and expensive exercise, since many factors influencing transmission behaviour have inter-related effects.
A particular transmission may have a well-developed shift map which meets circumstances of use. Such a shift map may not only control ratio selection and timing, but do so in several modes, such as ‘comfort’ and ‘sport’. The mating of such a transmission to an alternative engine or vehicle may reveal certain shortcomings, which do not however warrant a change to the current shift map. Where several alternative engines are provided, several different shift maps may be indicated, with a consequent potential increase in cost and complication.
On the other hand vehicle manufacturers would prefer to minimize the number of transmission variants, both to reduce manufacturing and stocking inventory, and to minimize the number of different types of transmission in service.
Where a large number of speed ratios is provided, it may be possible to safely start from rest in more than one speed ratio, depending on external ambient factors, conditions of vehicle use etc. A vehicle driver will select the appropriate ratio in a manual transmission according to circumstances and experience, and may choose or be authorized to launch in second speed rather than first (lowest) speed if conditions are appropriate.
In the case of an automatically controlled transmission, a suitable control strategy is required in order to determine the appropriate speed ratio, and in the case of a transmission with many speeds to determine the launch ratio.